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TMP451

SBOS686 – JUNE 2013

www.ti.com

TIMEOUT FUNCTION

The TMP451 resets the serial interface if either SCL or SDA are held low for 25 ms (typical) between a start and

stop condition. If the TMP451 is holding the bus low, the device releases the bus and waits for a start condition.

To avoid activating the timeout function, it is necessary to maintain a communication speed of at least 1 kHz for

the SCL operating frequency.

SHUTDOWN MODE (SD)

The TMP451 shutdown mode enables the user to save maximum power by shutting down all device circuitry

other than the serial interface, reducing current consumption to typically less than 3

μA; see Figure 10, Shutdown

Quiescent Current vs Supply Voltage. Shutdown mode is enabled when SD (bit 6) of configuration register is

high; the device shuts down after the current conversion is finished. When SD is low, the device maintains a

continuous-conversion state.

SENSOR FAULT

The TMP451 can sense a fault at the D+ input resulting from incorrect diode connection. The TMP451 can also

sense an open circuit. Short-circuit conditions return a value of –64°C. The detection circuitry consists of a

voltage comparator that trips when the voltage at D+ exceeds (V+) – 0.3 V (typical). The comparator output is

continuously checked during a conversion. If a fault is detected, then OPEN (bit 2) in the status register is set to

'1'.

When not using the remote sensor with the TMP451, the D+ and D– inputs must be connected together to

prevent meaningless fault warnings.

FILTERING

Remote junction temperature sensors are usually implemented in a noisy environment. Noise is most often

created by fast digital signals, and it can corrupt measurements. The TMP451 has a built-in, 65-kHz filter on the

inputs of D+ and D– to minimize the effects of noise. However, a bypass capacitor placed differentially across the

inputs of the remote temperature sensor is recommended to make the application more robust against unwanted

coupled signals. For this capacitor, choose a value of between 100 pF and 1 nF. Some applications attain better

overall accuracy with additional series resistance; however, this increased accuracy is application-specific. When

series resistance is added, the total value should not be greater than 1 k

Ω. If filtering is required, suggested

component values are 100 pF and 50

Ω on each input; exact values are application-specific.

Additionally, a digital filter is available for the remote temperature measurements to further reduce the effect of

noise. This filter is programmable and has two levels when enabled. Level 1 perfoms a moving average of four

consecutive samples. Level 2 performs a moving average of eight consecutive samples. The value stored in the

remote temperature result register is the output of the digital filter, and the ALERT and THERM limits are

compared to it. This provides additional immunity to noise and spikes on the ALERT and THERM outputs. The

filter responses are shown in Figure 18. The filter can be enabled or disabled by programming the desired levels

in the digital filter register. The digital filter is disabled by default and on POR.

Figure 18. Filter Response to Impulse and Step Inputs

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